This invention relates to a battery operated power supply for an electroluminescent (EL) lamp and, in particular, to a plurality of inverters operating in concert for powering an EL lamp.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by an inverter that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred and twenty volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the EL lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
The prior art discloses several types of inverters including an inductive boost circuit having an inductor in series with a switching transistor. The energy stored in the inductor when the transistor is conducting is supplied to an EL lamp as a small current at high voltage when the transistor stops conducting. The voltage on the lamp is pumped up by a series of high frequency pulses from the inverter. The direct current produced by the inverter must be converted into an alternating current in order to power an EL lamp. U.S. Pat. No. 4,527,096 (Kindlmann) discloses a switching bridge to alternate the current through the lamp. The bridge changes the polarity of the current through the lamp at a low frequency (200-1000 hertz). Another solution is disclosed in U.S. Pat. No. 5,349,269 (Kimball), wherein a pair of inverters are used to alternately power an EL lamp. A first inverter charges the lamp to a first polarity and the second inverter charges the lamp to the opposite polarity. Yet another solution is disclosed in U.S. Pat. No. 5,313,117 (Kimball), wherein the inverter produces an AC voltage between a single output terminal and ground.
In the prior art, most of the applications for EL lamps required a lamp having an area of one to three square inches. As a result, commercially available EL drivers are low power (.ltoreq.400 mW) devices. The SP4423 and SP4425 as sold by Sipex Corporation and the D358 inverter as sold by Durel Corporation are examples. There are many applications for EL lamps in which the area of the lamp exceeds five square inches. Existing inverters are not capable of providing sufficient power for these larger EL lamps.
In principle, adding power sources in parallel for increased current capacity is known. In practice, combining inverters has not been done, largely for economic reasons. The increased brightness comes at too high a cost.
U.S. Pat. No. 5,418,434 (Kamens et al.) discloses an inverter using two inductive boost circuits and discloses lower peak current drain compared to a conventional, single boost circuit. Average current, average power, and brightness are all the same as a single boost circuit. Alternately switching the inductors limits the duty cycle to .ltoreq.50%, reducing the efficiency of the individual boost circuits.
The EL lamp market is very cost sensitive and an implementation of the schematic shown in the Kamens et al. patent is likely to be more expensive than an inverter using a single boost circuit. In a boost circuit, the peak current determines the size (and cost) of the switching transistor. In the Kamens et al. circuit, if the peak current is reduced twenty-five percent, then the area of a switching transistor can be reduced a like amount. However, one needs two switching transistors. Thus, one actually needs about fifty percent more silicon (2.times.0.75=1.50) as for a boost circuit using a single transistor. Thus, the cost would be greater with no increase in brightness.
In view of the foregoing, it is therefore an object of the invention to provide an efficient power supply for driving large area lamps, e.g. lamps having an area greater than five square inches.
Another object of the invention is to provide a cost effective increase in brightness for low power inverters.
A further object of the invention is to provide a power supply for EL lamps having increased brightness.
Another object of the invention is to obtain an increased in brightness from a low power inverter without exceeding the power rating of the inverter.